Method of reducing solder wicking on a substrate

ABSTRACT

This invention relates to a substrate with via and pad structure(s) to reduce solder wicking. Each via and pad structure connects a component to conductive layers associated with the substrate. The substrate includes one or more plated vias, solder mask(s) surrounding the plated vias, and a conductive pad with a conductive trace connected to each plated via. The conductive pad extends beyond the terminal sides to increase solder formation and the solder mask reduces solder formation at the terminal end of the component. The via and pad structure is suitable for a variety of components and high component density. The invention also provides a computer implemented method for calculating the maximum distance of a conductive pad extending beyond the terminal side of a component.

This application is a divisional of U.S. application Ser. No.10/774,115, Substrate With Via And Pad Structures, filed on Feb. 6,2004, now U.S. Pat. No. 7,645,940 which is incorporated by referenceherein.

BACKGROUND

The present invention relates to a substrate with one or more via andpad structures for use in circuit assemblies with high reliability andhigh yield in manufacturing.

FIG. 1 illustrates a printed wiring board (PWB) with a substrate 10having several plated vias such as plated via 35. Each plated viaconnects to associated conductive layers of the substrate 10. Theconductive layers are located on the top, bottom, and/or in thesubstrate 10. Another structure is the via and pad structure such as viaand pad structure 20 shown without a component. Via and pad structure 20includes a solder mask 24, and a conductive trace 26 connecting aconductive pad 22 and a plated via 25 with a via hole 28.

FIG. 1 also illustrates that when conductive pad 32 is close to the viahole 38 the solder mask 34 will not prevent the solder from beingsiphoned into the via hole 38. The small amount of solder remaining willform an unreliable solder joint 31 at the terminal end 37 of thecomponent 33. This problem is referred to as solder wicking. Decreasingthe inner diameter of the solder mask 34 may reduce the solder wicking,but will require that the solder mask 34 be precisely aligned on theplated via 12 to avoid covering the via hole 38 and trappingcontaminants causing corrosion. Gold plating on the plated via 12 maydecrease corrosion but will increase solder wicking by increasing thewettability of the plated via 12. Solder wicking can also misalign acomponent with respect to a conductive pad such as terminal end 29 ofcomponent 13 which is misaligned with respect to the conductive pad 21.Another approach to reduce solder wicking is to move the conductive pad18 further from the via hole 8, but this requires a long conductivetrace, such as that covered by solder mask 17, which increases traceinductance and degrades performance at high frequencies.

FIG. 2A is a perspective view and FIG. 2B a sectional view of a surfacemount component 33 on a substrate 10. FIGS. 2A-2B illustrate the solderwicking problem that arises using conventional via and pad structure 30.The unreliable solder joint 31 is formed on the terminal end 37 of thecomponent 33 because the solder mask 34 is not effective due to thecapillary action of the via hole 38 in close proximity to the conductivepad 32.

SUMMARY OF THE INVENTION

The present invention relates to a substrate with via and pad structuresfor use in circuit assemblies. In an embodiment, the substrate has oneor more via and pad structures for connecting a component to conductivelayers associated with the substrate. The via and pad structure includesa plated via connected to associated conductive layers, a solder masksurrounding the plated via, and a conductive pad with a conductive traceconnected to the plated via, wherein the conductive pad extends beyondthe terminal sides of the component to increase solder formation at theterminal sides and the solder mask reduces solder formation at theterminal end of the component. The via and pad structures are applicableto the circuit layouts for a variety of components and high componentdensity can be achieved with reliable solder joints to the componentterminal sides rather than the terminal end. The invention also providesa method for calculating the distance the conductive pad extends beyondthe terminal side of a component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a PWB using conventional via and pad structures.

FIG. 2A is a perspective view of a component connected by a pair ofconventional via and pad structures.

FIG. 2B is a sectional view illustrating an unreliable solder jointformed on a terminal end of component when using a conventional via andpad structure.

FIG. 3 illustrates a PWB or printed circuit board using via and padstructures for mounting components.

FIG. 4A is a perspective view of a component mounted on a pair of viaand pad structures.

FIG. 4B is a sectional view illustrating the solder joint formed on theterminal sides of the component using the via and pad structures.

FIG. 5 illustrates the relationship, spacing and geometry of theconductive pads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description includes the best mode of carrying out theinvention. The detailed description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is determined byreference to the claims.

We assign each part, even if structurally identical to another part, aunique reference number wherever that part is shown in the drawingfigures. A dashed circle indicates part of a figure that is enlarged inanother figure. Any figure showing the enlarged part is indicated by areference number tied to the dashed circle.

FIG. 3 illustrates a substrate such as a PWB or a printed circuit boardwith via and pad structures for connecting surface mount components. Asurface mount component such as component 43 has plated terminals 82 and84 to form solder joints 3, 4, 5, and 6 with corresponding conductivepads 86 and 88. The substrate 80 may include conductive top, bottom,and/or embedded layers. The plated vias such as plated via 61, connectedto associated conductive layers, can be formed by drilling a hole insubstrate 80 then plating the hole to form a via hole 67.

Although components can be arranged along a column or row of the arrayof plated vias, the tight spacing in an array in a typical PWB limitsthe component to being placed diagonally as shown. A component 43representing, for example, a 0402 size (i.e., 0.04″ length×0.02″width×0.02″ height) chip capacitor, shown in the bottom right of FIG. 3,is oriented diagonally in a space 42, that is, between the keyholeshaped solder masks 7 and 9. This orientation keeps the conductive traceto a minimal length which minimizes trace inductance causing performancedegradation such as group delay, self-resonance with capacitivecomponents or frequency roll off.

In via and pad structure 60, the conductive pad 62 has a conductivetrace 63 (covered by the keyhole shaped solder mask 64) and connected toa plated via 65. The close proximity of the plated via 65 to theconductive pad 62 means the conductive trace 63 is short in length andhas low trace inductance. In the illustrated embodiment, the conductivepad 62 includes a T-shirt shaped structure of a first arm 91, a secondarm 92, and a body 93. Preferably, the body 93 extends beyond the length47 of the arms 91 and 92. The conductive pad 62 has arms 91 and 92 witha width 48 that extends beyond the terminal sides of the component (notshown). Via and pad structure 69 depicts an alternative embodiment,where conductive trace 90 connects the conductive pad 99 to the platedvia 73 and a ring shaped solder mask 59 acts as a solder barrier.Conductive pad 99 includes arms 94 and 95 but no body such as the body93 in conductive pad 62.

The extended conductive pad increases formation of a reliable solderjoint on each terminal side of the component. This is shown in via andpad structure 50, which has the solder joints 41, 51 on the terminalsides 75 and 76.

Each solder mask such as the keyhole shaped solder mask 54 is made ofnon-conductive and non-wettable material such as an epoxy, a photoimageable solder mask, or another suitable material. Solder masks suchas solder masks 54 and 64 are keyhole shaped and have a sufficient width44 to form a solder barrier despite the short length of the conductivetraces 56 and 63. For example, a 0402 component would overlay thekeyhole shaped solder mask 54. The keyhole shaped solder mask 54 is thesame shape as the keyhole shaped solder mask 64 and both have a typicalwidth 44 of 3.5 mil and preferably greater to reduce formation of asolder joint such as at the terminal end 57 of component 53, whichprevents solder wicking. Solder joints 41 and 51 form on the terminalsides 75 and 76 and the conductive pad 52. In an alternative embodiment,the solder mask 81 covers the substrate 80 partially or entirely exceptfor the conductive pads such as conductive pad 98 and the plated viaholes such as plated via hole 77.

In an embodiment, the via and pad structure 50 has a conductive pad 52that extends beyond the terminal sides 75 and 76 of the component 53 toincrease solder formation on arms 96 and 97. In addition, the arms 96and 97 are preferably symmetric to the plated via 55 on the substrate 80so the solder joints 41 and 51 will generate substantially equal surfacetension to the terminal sides 75 and 76 of the component 53. This keepsthe component 53 aligned with the conductive pads 52 and 98.

FIG. 4A is a perspective view illustrating a component 53 using a viaand pad structure 50. The keyhole shaped solder mask 54 surrounds theplated via 55 and reduces the formation of a solder joint at theterminal end 57. Solder joints 41 and 51 form on terminal sides 75 and76 as well as arms 96 and 97. Because the arms 96 and 97 aresubstantially equal areas the solder joints 41 and 51 generatesubstantially equal surface tension during solder reflow that keeps thesurface mount component 53 aligned with the conductive pad 52.

FIG. 4B is a sectional view of the component 53 shown in FIG. 4Aillustrating the solder joint 51 formed on the terminal side 75 using avia and pad structure 50 on substrate 80. The keyhole shaped solder mask54 reduces or prevents the formation of a solder joint at terminal end57 on the conductive pad 52 thus reducing solder wicking where thesolder crosses over the keyhole shaped solder mask 54 into the via hole58.

In another embodiment of the via and pad structure, the wettability ofthe plated via can be reduced by applying a chemically compatibleplating finish to the surface of the plated via such as a known organicoxidant surface finish or immersion silver. This reduces solder wickingduring the solder reflow process.

FIG. 5 illustrates the spatial relationship of the via and padstructures to provide a method of sizing the conductive pads 86 and 88that can be implemented in computer software. The method shows how tomaximize the distance the conductive pad extends beyond the terminalside to reduce solder wicking without generating any electrical shortsbetween the conductive pad and the adjacent plated via. The method alsooptimizes component density. As shown in FIG. 5, L1 is thecenter-to-center distance between the via hole 15 and the via hole 16.Component 43 has a length L3 and a width L4. L2 is the center-to-centerdistance of the via hole 14 and the via hole 16. L2 is also thehypotenuse of a square with sides of length L1. L5 is the length of theconductive pad 86 extending beyond the terminal side. L5 can be greaterthan or equal to the plated terminal 82 of component 43. L7 is half thedistance between the conductive pads 86 and 88 extending beyond theterminal side. L8 is the distance from the center of the via hole 16 tothe side of component 43. Radius R is outer radius of the plated via 19.Distance X is the minimum distance between the plated via 19 to theconductive pad 88. L11 is the sum of radius R and X. The distance X isset as a design rule to prevent solder wicking. A right triangle isformed by the sides L7, L9 and L11. Once L11 and L7 are known,Pythagorean theorem can be used to solve for L9. Subtracting L9 from L8gives us L10, which is the maximum distance of the conductive pad 88extending beyond the terminal side of the component 43.

For example, we can calculate the maximum distance L10 for a 0402component by using the following values: L1=39.39 mil (1 mm), L3=40 mil,L4=20 mil, L5=10 mil, R=9.5 mil, and X=5 mil.L2=L1/sin 45°=55.7 milL8=(L2−L4)/2=17.9 milL7=L3/2−L5=10 milL11=R+X=14.5 milL9=square root (L11² −L7²)=10.5 milL10=L8−L9=7.4 mil

L10 may be then rounded to 7 mil for the layout. This method can beimplemented in computer software where the variables are represented byvalues to solve other layouts for other component sizes.

1. A method of reducing solder wicking on a substrate when connecting asurface mount component to conductive layers of the substrate,comprising: providing a surface mount component that includes a packagehaving an upper surface with first solderable terminal sides and a firstterminal end and second solderable terminal sides and a second terminalend; connecting a first plated via to a first conductive layer;surrounding the first plated via with a first solder mask; connecting asecond plated via to a second conductive layer; surrounding the secondplated via to a second solder mask; connecting a first conductive padwith a conductive trace to the first plated via, wherein the firstconductive pad includes a portion that is exposed to solder and extendsbeyond the first solderable terminal sides of the surface mountcomponent to increase solder formation along the first solderableterminal sides and to reduce solder formation at the first plated via;connecting a second conductive pad with a conductive trace connected tothe second plated via, wherein the second conductive pad includes aportion that is exposed to solder and extends beyond the secondsolderable terminal sides of the surface mount component to increasesolder formation along the second solderable terminal sides and toreduce solder formation at the second plated via, wherein each of thefirst and second conductive pads is symmetric with respect to the firstplated via and the second plated via; reducing solder formation using afirst solder mask at the first terminal end of the surface mountcomponent; and reducing solder formation using a second solder mask atthe second terminal end of the surface mount component.
 2. A method ofreducing solder wicking on a substrate when connecting a surface mountcomponent to conductive layers of the substrate, comprising: providing asurface mount component that includes a package having an upper surfacewith first solderable terminal sides and a first terminal end and secondsolderable terminal sides and a second terminal end; connecting a firstplated via to a first conductive layer; surrounding the first plated viawith a first solder mask; connecting a second plated via to a secondconductive layer; surrounding the second plated via to a second soldermask; forming a first conductive pad and a second conductive pads toinclude respectively a first T-shirt shaped structure and a secondT-shirt shaped structure; connecting the first conductive pad with aconductive trace to the first plated via, wherein the first conductivepad includes a portion that is exposed to solder and extends beyond thefirst solderable terminal sides of the surface mount component toincrease solder formation along the first solderable terminal sides andto reduce solder formation at the first plated via; connecting thesecond conductive pad with a conductive trace connected to the secondplated via, wherein the second conductive pad includes a portion that isexposed to solder and extends beyond the second solderable terminalsides of the surface mount component to increase solder formation alongthe second solderable terminal sides and to reduce solder formation atthe second plated via; reducing solder formation using a first soldermask at the first terminal end of the surface mount component; andreducing solder formation using a second solder mask at the secondterminal end of the surface mount component.
 3. The method of claim 2,further comprising disposing the first and second T-shirt structuressymmetrically with respect to the first and second plated vias,respectively.
 4. A method of reducing solder wicking on a substrate whenconnecting a surface mount component to conductive layers of thesubstrate, comprising: providing a surface mount component that includesa package having an upper surface with first solderable terminal sidesand a first terminal end and second solderable terminal sides and asecond terminal end; connecting a first plated via to a first conductivelayer; surrounding the first plated via with a first solder mask;connecting a second plated via to a second conductive layer; surroundingthe second plated via to a second solder mask; connecting a firstconductive pad with a conductive trace to the first plated via, whereinthe first conductive pad includes a portion that is exposed to solderand extends beyond the first solderable terminal sides of the surfacemount component to increase solder formation along the first solderableterminal sides and to reduce solder formation at the first plated via;connecting a second conductive pad with a conductive trace connected tothe second plated via, wherein the second conductive pad includes aportion that is exposed to solder and extends beyond the secondsolderable terminal sides of the surface mount component to increasesolder formation along the second solderable terminal sides and toreduce solder formation at the second plated via; reducing solderformation using a first solder mask at the first terminal end of thesurface mount component; reducing solder formation using a second soldermask at the second terminal end of the surface mount component; andconnecting a surface mount component to the first and second conductivepads through solder joint(s) having a greater volume at each of thesolderable terminal sides than at each terminal end of the surface mountcomponent.